IPS Mallikarjun Chadalapaka
Internet Draft Rob Elliott
draft-ietf-ips-command-ordering-02.txt Hewlett-Packard Co.
Category: Informational-track
SCSI Command Ordering Considerations with iSCSI
Status of this Memo
This document is an Internet-Draft and fully conforms to all
provisions of Section 10 of RFC2026.
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Abstract
iSCSI is a SCSI transport protocol designed to run on top of
TCP. The iSCSI session abstraction is equivalent to the classic
SCSI "I_T nexus" which represents the logical relationship
between an Initiator and a Target (I and T) required in order to
communicate via the SCSI family of protocols. The iSCSI session
provides an ordered command delivery from the SCSI initiator to
the SCSI target. This document goes into the design
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considerations that led to the iSCSI session model as it is
defined today, relates the SCSI command ordering features
defined in T10 specifications to the iSCSI concepts, and finally
provides guidance to system designers on how true command
ordering solutions can be built based on iSCSI.
Acknowledgments
We are grateful to the IPS working group whose work defined the
iSCSI protocol. Thanks also to David Black (EMC) who encouraged
the publication of this document. Special thanks to Randy
Haagens (HP) for his insights on the topic of command ordering.
Thanks are also due to Elizabeth Rodriguez for carefully
reviewing this document.
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Status of this Memo . . . . . . . . . . . . . . . . . . . . . . . . . 1
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions and Acronyms . . . . . . . . . . . . . . . . . . . . . 5
2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview of the iSCSI Protocol . . . . . . . . . . . . . . . . . . 7
3.1 Protocol Mapping Description . . . . . . . . . . . . . . . . . . 7
3.2 The I_T Nexus Model . . . . . . . . . . . . . . . . . . . . . . 7
3.3 Ordered Command Delivery . . . . . . . . . . . . . . . . . . . . 9
3.3.1 Questions . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.2 The Session Guarantee . . . . . . . . . . . . . . . . . . . 9
3.3.3 Ordering Onus . . . . . . . . . . . . . . . . . . . . . . .10
3.3.4 Design Intent . . . . . . . . . . . . . . . . . . . . . . .10
4. The Command Ordering Scenario . . . . . . . . . . . . . . . . . . .11
4.1 SCSI Layer . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4.1.1 Command Reference Number (CRN) . . . . . . . . . . . . . .11
4.1.2 Task Attributes . . . . . . . . . . . . . . . . . . . . . .11
4.1.3 Auto Contingent Allegiance (ACA) . . . . . . . . . . . . .11
4.1.4 UA Interlock . . . . . . . . . . . . . . . . . . . . . . .12
4.2 iSCSI Layer . . . . . . . . . . . . . . . . . . . . . . . . . .12
5. Connection Failure Considerations . . . . . . . . . . . . . . . . .13
6. Command Ordering System Considerations . . . . . . . . . . . . . .14
7. Reservation Considerations . . . . . . . . . . . . . . . . . . . .15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . .16
9. Security Considerations . . . . . . . . . . . . . . . . . . . . . .17
10. References and Bibliography . . . . . . . . . . . . . . . . . . .18
10.1 Normative References . . . . . . . . . . . . . . . . . . . . .18
10.2 Informative References: . . . . . . . . . . . . . . . . . . . .18
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .19
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . .20
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1. Introduction
iSCSI is a SCSI transport protocol ([iSCSI]) designed to enable
running SCSI application protocols on TCP/IP networks, including
potentially the Internet. Given the size and scope of Internet,
iSCSI thus enables some exciting new SCSI applications.
Potential new application areas for exploiting iSCSI's value
include the following.
a) Larger (diameter) Storage Area Networks (SANs) than
had been possible until now.
b) Asynchronous remote mirroring
c) Remote tape vaulting
Each of these applications takes advantage of the practically
unlimited geographical distance that iSCSI enables between a
SCSI initiator and a SCSI target. In each of these cases,
because of the long delays involved, there is a very high
incentive for the initiator to stream SCSI commands back-to-back
without waiting for the SCSI status of previous commands.
Command streaming may be employed primarily by two classes of
applications - while one class may not particularly care about
ordered command execution, the other class does rely on ordered
command execution (i.e. there is an application-level dependency
on the ordering among SCSI commands). As an example, cases b)
and c) listed earlier clearly require ordered command execution.
A mirroring application does not want the writes to be committed
out of order on the remote SCSI target, so as to preserve the
transactional integrity of the data on that target. To
summarize, SCSI command streaming when coupled with the
guarantee of ordered command execution on the SCSI target is
extremely valuable for a critical class of applications in long-
latency networks.
This document reviews the various protocol considerations in
designing storage solutions that employ SCSI command ordering.
This document also analyzes and explains the design intent of
[iSCSI] with respect to command ordering.
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2. Definitions and Acronyms
2.1 Definitions
- I_T nexus: [SAM2] defines the I_T nexus as a relationship
between a SCSI initiator port and a SCSI target port. [iSCSI]
defines an iSCSI session as the iSCSI representation of an I_T
nexus. In the iSCSI context, the I_T nexus (i.e. the iSCSI
session) is a relationship between an iSCSI initiator's end of
the session (SCSI Initiator Port) and the iSCSI target's Portal
Group (SCSI Target Port).
- PDU (Protocol Data Unit): An iSCSI initiator and iSCSI target
communicate using iSCSI protocol messages. These messages are
called "iSCSI protocol data units" (iSCSI PDUs).
- SCSI device: A SCSI device is an entity that contains one or
more SCSI ports that are connected to a service delivery
subsystem and supports SCSI application protocols. In the iSCSI
context, the SCSI Device is the component within an iSCSI Node
that provides the SCSI functionality. The SCSI Device Name is
defined to be the iSCSI Name of the node.
- Session: A group of logically related iSCSI connections that
link an initiator with a target form a session (equivalent to a
SCSI I-T nexus). The number of participating iSCSI connections
within an iSCSI session may vary over time. The multiplicity of
connections at the iSCSI level is completely hidden for the SCSI
layer - each SCSI port in an I_T nexus sees only one peer SCSI
port across all the connections of a session.
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2.2 Acronyms
Acronym Definition
--------------------------------------------------------------
ACA Auto Contingent Allegiance
ASC Additional Sense Code
ASCQ Additional Sense Code Qualifier
CRN Command Reference Number
IETF Internet Engineering Task Force
ISID Initiator Session Identifier
ITT Initiator Task Tag
LU Logical Unit
LUN Logical Unit Number
NIC Network Interface Card
PDU Protocol Data Unit
TMF Task Management Function
TSIH Target Session Identifying Handle
SAM-2 SCSI Architecture Model - 2
SAN Storage Area Network
SCSI Small Computer Systems Interface
TCP Transmission Control Protocol
UA Unit Attention
WG Working Group
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3. Overview of the iSCSI Protocol
3.1 Protocol Mapping Description
The iSCSI protocol is a mapping of the SCSI remote procedure
invocation model (see [SAM2]) over the TCP protocol.
SCSI's notion of a task maps to an iSCSI task. Each iSCSI task
is uniquely identified within that I_T nexus by a 32-bit unique
identifier called Initiator Task Tag (ITT). The ITT is both an
iSCSI identifier of the task and a classic SCSI task tag.
SCSI commands from the initiator to the target are carried in
iSCSI requests called SCSI Command PDUs. SCSI status back to
the initiator is carried in iSCSI responses called SCSI Response
PDUs. SCSI Data-out from the initiator to the target is carried
in SCSI Data-Out PDUs, and the SCSI Data-in back to the
initiator is carried in SCSI Data-in PDUs.
3.2 The I_T Nexus Model
In the iSCSI model, the SCSI I_T nexus maps directly to the
iSCSI session which is an iSCSI protocol abstraction spanning
one or more TCP connections. The iSCSI protocol defines the
semantics in order to realize one logical flow of bidirectional
communication on the I_T nexus potentially spanning multiple TCP
connections (as many as 2^16). The multiplicity of iSCSI
connections is thus completely contained at the iSCSI layer,
while the SCSI layer is presented with a single I_T nexus even
in a multi-connection session. A session between a pair of
given iSCSI nodes is identified by the session identifier (SSID)
and each connection within a given session is uniquely
identified by a connection identifier (CID) in iSCSI. The SSID
itself has two components - Initiator Session Identifier (ISID)
and a Target Session Identifying Handler (TSIH) - each
identifying one end of the same session.
There are four crucial functional facets of iSCSI that together
present this single logical flow abstraction to the SCSI layer
even with an iSCSI session spanning across multiple iSCSI
connections.
a) Ordered command delivery: A sequence of SCSI commands
that is striped across all the connections in the
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session is "reordered" by the target iSCSI layer into
an identical sequence based on a Command Sequence
Number (CmdSN) that is unique across the session. The
goal is to achieve bandwidth aggregation from multiple
TCP connections, but to still make it appear to the
target SCSI layer as if all the commands had travelled
in one flow.
b) Connection allegiance: All the PDU exchanges for a
SCSI Command, up to and including the SCSI Response PDU
for the Command, are required to flow on the same iSCSI
connection at any given time. This again is intended
to hide the multi-connection nature of a session
because the SCSI layer on either side will never see
the PDU contents out of order (e.g., status cannot
bypass read data for an initiator).
c) Task set management function handling: [iSCSI]
specifies an ordered sequence of steps for the iSCSI
layer on the SCSI target in handling the two SCSI task
management functions (TMFs) that manage SCSI task sets.
The two TMFs are ABORT TASK SET that aborts all active
tasks in a session and CLEAR TASK SET that clears the
tasks in the task set. The goal of the sequence of
steps is to guarantee that the initiator receives the
SCSI Response PDUs of all unaffected tasks before the
TMF Response itself arrives, regardless of the number
of connections in the iSCSI session. This operational
model is again intended to preserve the single flow
abstraction to the SCSI layer.
d) Immediate task management function handling: Even when
a TMF request is marked as "immediate" (i.e. only has a
position in the command stream, but does not consume a
CmdSN), [iSCSI] defines semantics that require the
target iSCSI layer to ensure that the TMF request is
executed as if the commands and the TMF request were
all flowing on a single logical channel. This ensures
that the TMF request will act on tasks that it was
meant to manage.
The following sections will analyze the "Ordered command
delivery" aspect in more detail, since command ordering is the
focus of this document.
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3.3 Ordered Command Delivery
3.3.1 Questions
A couple of important questions related to iSCSI command
ordering were considered early on in the design of the iSCSI
protocol. The questions were:
a) What should be the command ordering behavior required
of iSCSI implementations in the presence of transport
errors, such as errors that corrupt the data in a
fashion that is not detected by the TCP checksum (e.g.,
two offsetting bit flips in the same bit position), but
is detected by the iSCSI CRC digest?
b) Should [iSCSI] require both initiators and targets to
use ordered command delivery?
Since the answers to these questions are critical to the
understanding of the ordering behavior required by the iSCSI
protocol, the following sub-sections consider them in more
detail.
3.3.2 The Session Guarantee
The final disposition of question a) in section 3.3.1 was
reflected in [RFC3347], "iSCSI MUST specify strictly ordered
delivery of SCSI commands over an iSCSI session between an
initiator/target pair, even in the presence of transport
errors.". Stated differently, an iSCSI digest failure, or an
iSCSI connection termination must not cause the iSCSI layer on a
target to allow executing the commands in an order different
from that intended (as indicated by the CmdSN order) by the
initiator. This design choice is enormously helpful in building
storage systems and solutions that can now always assume command
ordering to be a service characteristic of an iSCSI substrate.
Note that by taking the position that an iSCSI session always
guarantees command ordering, [iSCSI] was indirectly implying
that the principal reason for the multi-connection iSCSI session
abstraction was to allow ordered bandwidth aggregation for an
I_T nexus. In deployment models where this cross-connection
ordering mandated by [iSCSI] is deemed expensive, a serious
consideration should be given to deploying multiple single-
connection sessions in stead.
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3.3.3 Ordering Onus
The final resolution of b) in section 3.3.1 by the iSCSI
protocol designers was in favor of not requiring the initiators
to use command ordering always. This resolution is reflected in
dropping the mandatory ACA usage requirement on the initiators,
and allowing an ABORT TASK TMF to plug a command hole etc.,
since these are conscious choices an initiator makes in favor of
not using ordered command delivery. The net result can be
discerned by a careful reader of [iSCSI] - the onus of ensuring
ordered command delivery is always on the iSCSI targets, while
the initiators may or may not utilize command ordering. iSCSI
targets being the servers in the client-server model do not
really attempt to establish whether or not a client (initiator)
intends to take advantage of command ordering service, but
instead simply always provide the guaranteed delivery service.
The rationale here is that there are inherent SCSI and
application-level dependencies as we shall see in building a
command ordered solution that are beyond the scope of [iSCSI],
to mandate or even discern the intent with respect to the usage
of command ordering.
3.3.4 Design Intent
To summarize the design intent of [iSCSI] -
The service delivery subsystem (see [SAM2]) abstraction
provided by an iSCSI session is guaranteed to have the intrinsic
property of ordered delivery of commands to the target SCSI
layer under all conditions. Consequently, the guarantee of the
ordered command delivery is across the entire I_T nexus spanning
all the LUs that the nexus is authorized to access. It is the
initiator's discretion to whether or not make use of this
property.
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4. The Command Ordering Scenario
A storage systems designer working with SCSI and iSCSI has to
consider the following protocol features in SCSI and iSCSI
layers, each of which has a role to play in realizing the
command ordering goal.
4.1 SCSI Layer
The SCSI application layer has several tools to enforce
ordering.
4.1.1 Command Reference Number (CRN)
CRN is an ordered sequence number which when enabled for a
device server, increments by one for each I_T_L nexus (see
[SAM2]). The one notable drawback with CRN is that there is no
SCSI-generic way (such as through mode pages) to enable or
disable the CRN feature. [SAM2] also leaves the usage semantics
of CRN for the SCSI transport protocol, such as iSCSI, to
specify. [iSCSI] chose not to support the CRN feature for
various reasons.
4.1.2 Task Attributes
SAM-2 defines the following four task attributes - SIMPLE,
ORDERED, HEAD OF QUEUE, and ACA. Each task to an LU may be
assigned an attribute. [SAM2] defines the ordering constraints
that each of these attributes conveys to the device server that
is servicing the task. In particular, judicious use of ORDERED
and SIMPLE attributes applied to a stream of pipelined commands
could convey the precise execution schema for the commands that
the initiator issues, provided the commands are received in the
same order on the target.
4.1.3 Auto Contingent Allegiance (ACA)
ACA is an LU-level condition that is triggered when a command
(with the NACA bit set to 1) completes with CHECK CONDITION.
When ACA is triggered, it prevents all commands other than those
with the ACA attribute from executing until the CLEAR ACA task
management function is executed, while blocking all the other
tasks already in the task set. See [SAM2] for the detailed
semantics of ACA. Since ACA is closely tied to the notion of a
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task set, one would ideally have to select the scope of the task
set (by setting the TST bit to 1 in the control mode page of the
LU) to be per-initiator in order to prevent command failures in
one I_T_L nexus from impacting other I_T_L nexuses through ACA.
4.1.4 UA Interlock
When UA interlock is enabled, the logical unit does not clear
any standard Unit Attention condition reported with autosense
and in addition, establishes a Unit Attention condition when a
task is terminated with one of BUSY, TASK SET FULL, or
RESERVATION CONFLICT statuses. This so-called "interlocked UA"
is cleared only when the device server executes an explicit
REQUEST SENSE ([SPC3]) command from the same initiator. From a
functionality perspective, the scope of UA interlock today is
slightly different from ACA's because it enforces ordering
behavior for completion statuses other than CHECK CONDITION, but
otherwise conceptually has the same design intent as ACA. On
the other hand, ACA is somewhat more sophisticated because it
allows special "cleanup" tasks (ones with ACA attribute) to
execute when ACA is active. One of the principal reasons UA
interlock came into being was that SCSI designers wanted a
command ordering feature without the side effects of using the
aforementioned TST bit in the control mode page.
4.2 iSCSI Layer
As noted in section 3.2 and section 3.3, the iSCSI protocol
enforces and guarantees ordered command delivery per iSCSI
session using the CmdSN, and this is an attribute of the SCSI
transport layer. Note further that any command ordering
solution that seeks to realize ordering from the initiator SCSI
layer to the target SCSI layer would be of practical value only
when the command ordering is guaranteed by the SCSI transport
layer. In other words, the related SCSI application layer
protocol features such as ACA etc. are based on the premise of
an ordered SCSI transport. Thus iSCSI's command ordering is the
last piece in completing the puzzle of building solutions that
rely on ordered command execution, by providing the crucial
guarantee that all the commands handed to the initiator iSCSI
layer will be transported and handed to the target SCSI layer in
the same order.
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5. Connection Failure Considerations
[iSCSI] mandates that when an iSCSI connection fails, the active
tasks on that connection must be terminated if not recovered
within a certain negotiated time limit. When an iSCSI target
does terminate some subset of tasks due to iSCSI connection
dynamics, there is a danger that the SCSI layer would simply
move on to the next tasks waiting to be processed and execute
them out-of-order unbeknownst to the initiator SCSI layer. To
preclude this danger, [iSCSI] further mandates the following -
a) The tasks terminated due to the connection failure must
be internally terminated by the iSCSI target "as if" due to a
CHECK CONDITION. While this particular completion status is
never communicated back to the initiator, the "as if" is
still meaningful and required because if the initiator were
using ACA as the command ordering mechanism of choice, a
SCSI-level ACA will be triggered due to this mandatory CHECK
CONDITION. This addresses the aforementioned danger.
b) After the tasks are terminated due to the connection
failure, the iSCSI target must report a Unit Attention
condition on the next command processed on any connection for
each affected I_T_L nexus of that session. This is required
because if the initiator were using UA interlock as the
command ordering mechanism of choice, a SCSI-level UA will
trigger a UA-interlock. This again addresses the
aforementioned danger. iSCSI targets must report this UA with
the status of CHECK CONDITION, and the ASC/ASCQ value of 47h/
7Fh ("SOME COMMANDS CLEARED BY ISCSI PROTOCOL EVENT").
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6. Command Ordering System Considerations
In general, command ordering is automatically enforced if
targets and initiators comply with the iSCSI specification.
However, listed below are certain additional related
implementation considerations for the iSCSI initiators and
targets to take note of.
a) Even when all iSCSI and SCSI command ordering
considerations earlier noted in this draft were applied, it
is beneficial for iSCSI initiators to proactively avoid
scenarios that would otherwise lead to out-of-order command
execution. This is simply because the SCSI command ordering
features such as UA interlock are likely to be costlier in
performance when they are allowed to be triggered. [iSCSI]
provides enough guidance on how to implement this proactive
detection of PDU ordering errors.
b) The whole notion of command streaming does of course
assume that the target in question supports command queueing.
An iSCSI target desirous of supporting command ordering
solutions should ensure that the SCSI layer on the target
supports command queuing. Especially the remote backup (tape
vaulting) applications that iSCSI enables make a compelling
case that tape devices should give a very serious
consideration to supporting command queuing, at least when
used in conjunction with iSCSI.
c) An iSCSI target desirous of supporting high-performance
command ordering solutions that involve specifying a
description of execution schema should ensure that the SCSI
layer on the target in fact does support the ORDERED and
SIMPLE task attributes.
d) There is some consideration of expanding the scope of UA
interlock to encompass CHECK CONDITION status and thus make
it the only required command ordering functionality of
implementations to build command ordering solutions. Until
this is resolved in T10, the currently defined semantics of
UA interlock and ACA warrant implementing both features by
iSCSI targets desirous of supporting command ordering
solutions.
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7. Reservation Considerations
[iSCSI] describes a "principle of conservative reuse" that
encourages iSCSI initiators to reuse the same ISIDs (see section
3.2) to various SCSI target ports, in order to present the same
SCSI initiator port name to those target ports. This is in fact
a very crucial implementation consideration that must be
complied with. [SPC3] mandates the SCSI targets to associate
persistent reservations and the related registrations with the
SCSI initiator port names whenever they are required by the SCSI
transport protocol. Since [iSCSI] requires the mandatory SCSI
initiator port names based on ISIDs, iSCSI targets are required
to work off the SCSI initiator port names and thus indirectly
the ISIDs in enforcing the persistent reservations.
This fact has the following implications for the
implementations.
a) If a persistent reservation/registration is intended to
be used across multiple SCSI ports of a SCSI device, the
initiator iSCSI implementation must use the same ISID across
associated iSCSI sessions connecting to different iSCSI
target portal groups of the SCSI device.
b) If a persistent reservation/registration is intended to
be used across the power loss of a SCSI target, the initiator
iSCSI implementation must use the same ISIDs as before in re-
establishing the associated iSCSI sessions upon subsequent
reboot in order to rely on the persist through power loss
capability.
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8. IANA Considerations
This document does not have any IANA considerations.
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9. Security Considerations
For security considerations in using the iSCSI protocol, refer
to the Security Considerations section in [iSCSI]. This
document does not introduce any additional secuirty
considerations other than those already discussed in [iSCSI].
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10. References and Bibliography
10.1 Normative References
[iSCSI] J. Satran et. al. draft-ietf-ips-iscsi-20.txt
(work in progress)
[SAM] ANSI X3.270-1998, SCSI-3 Architecture Model (SAM).
[SAM2] T10/1157D, SCSI Architecture Model - 2 (SAM-2).
[SBC] NCITS.306-1998, SCSI-3 Block Commands (SBC).
10.2 Informative References:
[RFC793] TRANSMISSION CONTROL PROTOCOL, DARPA INTERNET
PROGRAM PROTOCOL SPECIFICATION, September 1981.
[RFC2119] Bradner, S. "Key Words for use in RFCs to Indi-
cate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3347] M. Krueger et. al., "iSCSI Requirements and
Design Considerations"
[SPC3]T10/1416-D, SCSI Primary Commands-3.
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11. Authors' Addresses
Mallikarjun Chadalapaka
Hewlett-Packard Company
8000 Foothills Blvd.
Roseville, CA 95747-5668, USA
Phone: +1.916.785.5621
E-mail: cbm@rose.hp.com
Rob Elliott
Hewlett-Packard Company
MC 150801
PO Box 692000
Houston, TX 77269-2000 USA
Phone: +1.281.518.5037
E-mail: elliott@hp.com
Comments may be sent to Mallikarjun Chadalapaka.
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